Resistor values are typically 120 to 240 ohms for 3.3V operation and 82 to 120 ohms for 2.5V
operation.
Figure 7 - Pull Up Pull Down Termination
Resistor values are typically for 3.3V operation
For 2.5V operation, the resistor to ground is 62 ohms and the resistor to supply is 240 ohms
There are numerous application notes on terminating and interfacing PECL logic and the two most common methods are a single resistor to ground, Figure 6,
and a pull-up/pull-down scheme as shown in Figure 7. An AC coupling capacitor is optional, depending on the application and the input logic requirements of
the next stage.
One of the most important considerations is terminating the Output and Complementary Outputs equally. An unused output should not be left un-
terminated, and if one of the two outputs is left open it will result in excessive jitter on both. PC board layout must take this and 50 ohm impedance matching
into account. Load matching and power supply noise are the main contributors to jitter related problems.
LVDS Application Diagrams
+3.3V
OE or NC
10nF
1nF
1
6
Receiver
100
3
4
+3.3V
OE or NC
10nF
1nF
1
6
10nF
100
3
4
Receiver
OE or NC
2
5
OE or NC
2
5
10nF
Figure 8 - LVDS to LVDS, internal 100Ω
Some LVDS structures have an internal 100 ohm resistor on the
input and do not need additional components.
Figure 9 - LVDS to LVDS, External 100Ω and AC block ing caps
One of the most important considerations is terminating the Output and Complementary Outputs equally. An unused output should not be left un-
terminated, and if one of the two outputs is left open it will result in excessive jitter on both. PC board layout must take this and 50 ohm impedance matching
into account. Load matching and power supply noise are the main contributors to jitter related problems.
FPGA/CPLD
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